The invention relates to improvements in rolls for use in calenders and like machines wherein webs of paper, plastic, textile or other material are caused to advance through the nip or nips of one or more pairs of rolls. More particularly, the invention relates to improvements in heatable rolls for use in calenders and like machines.
Commonly owned U.S. Pat. No. 4,757,584 to Pav et al. discloses a roll wherein the internal space of a hollow cylindrical shell, which is rotatable about a stationary carrier, receives at least two sets of hydrostatic bearing elements which are installed between two inserts. The inserts include antifriction bearings for the end portions of the shell, and twin sealing elements which prevent lubricant for the antifriction bearings from reaching the hydraulic fluid flowing to, in and from the hydrostatic bearing elements between the inserts. The median portion of the shell can be heated by such hydraulic fluid. The purpose of hydrostatic bearing elements is to permit adjustments of the width of the nip or nips which the cylindrical external surface of the roll defines with one or more neighboring rolls. An advantage of the patented roll is that hydraulic fluid which is used to change the configuration of the roll (by admitting pressurized hydraulic fluid to selected bearing elements and/or by permitting hydraulic fluid to escape from selected bearing elements) also forms part of the means for heating the median portion of the shell, either alone or in conjunction with additional heating means. The useful length of the external surface of the roll is that between the two inserts. In other words, the useful portion of the nip or nips between the patented roll and one or more adjacent rolls does not exceed the distance between the inserts in the end portion of the shell.
It is often necessary to heat the fluid which is supplied to the hydrostatic bearing elements to an elevated temperature in the range of 250.degree. to 350.degree. C. When the median portion of the shell is heated by a hydraulic fluid which is supplied at such elevated temperatures, the effective width of the nip or nips which are defined by the external surface of the shell is even less than the distance between the two inserts. The reason is that the temperature of the shell decreases rather abruptly in regions which are adjacent the inserts. Consequently, the actual length of the roll must greatly exceed the effective width of the nip or nips, and this contributes significantly to space requirements and initial and maintenance cost of the roll as well as to the space requirements and cost of the entire machine in which the roll is put to use. This also necessitates the use of a frame wherein the two lateral frame members are disposed at a considerable distance from each other, especially if the machine embodying the roll is used for the treatment of wide or extremely wide webs of paper, plastic material, fabric or the like. The cost of regulating the configuration of the shell in a heated calender roll increases with the length of the roll, and the accuracy of regulation is also affected if the roll must employ a long shell because only the median portion of the shell can be heated with a requisite degree of predictability. Moreover, a calender or a like machine with long or extremely long rolls cannot be readily installed in many existing plants.